Showing papers on "Excited state published in 2010"

Fluorescence Lifetime Measurements and Biological Imaging

Abstract: When a molecule absorbs a photon of appropriate energy, a chain of photophysical events ensues, such as internal conversion or vibrational relaxation (loss of energy in the absence of light emission), fluorescence, intersystem crossing (from singlet state to a triplet state) and phosphorescence, as shown in the Jablonski diagram for organic molecules (Fig. 1). Each of the processes occurs with a certain probability, characterized by decay rate constants (k). It can be shown that the average length of time τ for the set of molecules to decay from one state to another is reciprocally proportional to the rate of decay: τ = 1/k. This average length of time is called the mean lifetime, or simply lifetime. It can also be shown that the lifetime of a photophysical process is the time required by a population of N electronically excited molecules to be reduced by a factor of e. Correspondingly, the fluorescence lifetime is the time required by a population of excited fluorophores to decrease exponentially to N/e via the loss of energy through fluorescence and other non-radiative processes. The lifetime of photophycal processes vary significantly from tens of femotoseconds for internal conversion1,2 to nanoseconds for fluorescence and microseconds or seconds for phosphorescence.1 Open in a separate window Figure 1 Jablonski diagram and a timescale of photophysical processes for organic molecules.

Blue Luminescence of ZnO Nanoparticles Based on Non‐Equilibrium Processes: Defect Origins and Emission Controls

Abstract: High concentrations of defects are introduced into nanoscale ZnO through non-equilibrium processes and resultant blue emissions are comprehensively analyzed, focusing on defect origins and broad controls. Some ZnO nanoparticles exhibit very strong blue emissions, the intensity of which first increase and then decrease with annealing. These visible emissions exhibit strong and interesting excitation dependences: 1) the optimal excitation energy for blue emissions is near the bandgap energy, but the effective excitation can obviously be lower, even 420 nm (2.95 eV < Eg = 3.26 eV); in contrast, green emissions can be excited only by energies larger than the bandgap energy; and, 2) there are several fixed emitting wavelengths at 415, 440, 455 and 488 nm in the blue wave band, which exhibit considerable stability in different excitation and annealing conditions. Mechanisms for blue emissions from ZnO are proposed with interstitial-zinc-related defect levels as initial states. EPR spectra reveal the predominance of interstitial zinc in as-prepared samples, and the evolutions of coexisting interstitial zinc and oxygen vacancies with annealing. Furthermore, good controllability of visible emissions is achieved, including the co-emission of blue and green emissions and peak adjustment from blue to yellow.

Reducing Graphene Oxide on a Visible-Light BiVO4 Photocatalyst for an Enhanced Photoelectrochemical Water Splitting

Abstract: Bismuth vanadate (BiVO4) is incorporated with reduced graphene oxide (RGO) using a facile single-step photocatalytic reaction to improve its photoresponse in visible light. Remarkable 10-fold enhancement in photoelectrochemical water splitting reaction is observed on BiVO4−RGO composite compared with pure BiVO4 under visible illumination. This improvement is attributed to the longer electron lifetime of excited BiVO4 as the electrons are injected to RGO instantly at the site of generation, leading to a minimized charge recombination. Improved contact between BiVO4 particles with transparent conducting electrode using RGO scaffold also contributes to this photoresponse enhancement.

Conical intersection dynamics of the primary photoisomerization event in vision

Abstract: The primary photochemical event in vision, isomerization of the 11-cis chromophore in rhodopsin to the all-trans form, is one of the fastest natural photochemical processes known, taking less than a millionth of a millionth of a second. The molecular details of reactions of such rapidity are a stiff challenge to experimenters, but Polli et al. now report the characterization of the reaction using ultrafast optical spectroscopy with sub-20-femtosecond time resolution and spectral coverage from the visible to the near infrared. The data confirm that rhodopsin's extreme reactivity results from a molecular funnel mechanism that involves a 'conical intersection' between the potential energy surfaces of the starting and product molecules. Chemical reactions are usually described in terms of the movement of nuclei between the potential energy surfaces of ground and excited electronic states. Crossings known as conical intersections permit efficient transitions between the surfaces. It is shown here that ultrafast optical spectroscopy, with sub-20-fs time resolution and spectral coverage from the visible to the near-infrared, can map the isomerization of rhodopsin with sufficient resolution to shown that a conical intersection is important in this crucial event in vision. Ever since the conversion of the 11-cis retinal chromophore to its all-trans form in rhodopsin was identified as the primary photochemical event in vision1, experimentalists and theoreticians have tried to unravel the molecular details of this process. The high quantum yield of 0.65 (ref. 2), the production of the primary ground-state rhodopsin photoproduct within a mere 200 fs (refs 3–7), and the storage of considerable energy in the first stable bathorhodopsin intermediate8 all suggest an unusually fast and efficient photoactivated one-way reaction9. Rhodopsin's unique reactivity is generally attributed to a conical intersection between the potential energy surfaces of the ground and excited electronic states10,11 enabling the efficient and ultrafast conversion of photon energy into chemical energy12,13,14,15,16. But obtaining direct experimental evidence for the involvement of a conical intersection is challenging: the energy gap between the electronic states of the reacting molecule changes significantly over an ultrashort timescale, which calls for observational methods that combine high temporal resolution with a broad spectral observation window. Here we show that ultrafast optical spectroscopy with sub-20-fs time resolution and spectral coverage from the visible to the near-infrared allows us to follow the dynamics leading to the conical intersection in rhodopsin isomerization. We track coherent wave-packet motion from the photoexcited Franck–Condon region to the photoproduct by monitoring the loss of reactant emission and the subsequent appearance of photoproduct absorption, and find excellent agreement between the experimental observations and molecular dynamics calculations that involve a true electronic state crossing. Taken together, these findings constitute the most compelling evidence to date for the existence and importance of conical intersections in visual photochemistry.

Entanglement of Two Individual Neutral Atoms Using Rydberg Blockade

Abstract: We report the generation of entanglement between two individual $^{87}\mathrm{Rb}$ atoms in hyperfine ground states $|F=1,M=1⟩$ and $|F=2,M=2⟩$ which are held in two optical tweezers separated by $4\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$. Our scheme relies on the Rydberg blockade effect which prevents the simultaneous excitation of the two atoms to a Rydberg state. The entangled state is generated in about 200 ns using pulsed two-photon excitation. We quantify the entanglement by applying global Raman rotations on both atoms. We measure that 61% of the initial pairs of atoms are still present at the end of the entangling sequence. These pairs are in the target entangled state with a fidelity of 0.75.

Dynamics of a quantum phase transition and relaxation to a steady state

Abstract: We review recent theoretical work on two closely related issues: excitation of an isolated quantum condensed matter system driven adiabatically across a continuous quantum phase transition or a gapless phase, and apparent relaxation of an excited system after a sudden quench of a parameter in its Hamiltonian. Accordingly, the review is divided into two parts. The first part revolves around a quantum version of the Kibble–Zurek mechanism including also phenomena that go beyond this simple paradigm. What they have in common is that excitation of a gapless many-body system scales with a power of the driving rate. The second part attempts a systematic presentation of recent results and conjectures on apparent relaxation of a pure state of an isolated quantum many-body system after its excitation by a sudden quench. This research is motivated in part by recent experimental developments in the physics of ultracold atoms with potential applications in the adiabatic quantum state preparation and quantum computation.

Cooperative atom-light interaction in a blockaded Rydberg ensemble.

Abstract: By coupling a probe transition to a Rydberg state using electromagnetically induced transparency (EIT) we map the strong dipole-dipole interactions onto an optical field. We characterize the resulting cooperative optical nonlinearity as a function of probe strength and density. We demonstrate good quantitative agreement between the experiment and an $N$-atom cooperative model for $N=3$ atoms per blockade sphere and the $n=60$ Rydberg state. The measured linewidth of the EIT resonance places an upper limit on the dephasing rate of the blockade spheres of $l110\text{ }\text{ }\mathrm{kHz}$.

E-Type Delayed Fluorescence of a Phosphine-Supported Cu2(μ-NAr2)2 Diamond Core: Harvesting Singlet and Triplet Excitons in OLEDs∥

Abstract: A highly emissive bis(phosphine)diarylamido dinuclear copper(I) complex (quantum yield = 57%) was shown to exhibit E-type delayed fluorescence by variable temperature emission spectroscopy and photoluminescence decay measurement of doped vapor-deposited films. The lowest energy singlet and triplet excited states were assigned as charge transfer states on the basis of theoretical calculations and the small observed S_1−T_1 energy gap. Vapor-deposited OLEDs doped with the complex in the emissive layer gave a maximum external quantum efficiency of 16.1%, demonstrating that triplet excitons can be harvested very efficiently through the delayed fluorescence channel. The function of the emissive dopant in OLEDs was further probed by several physical methods, including electrically detected EPR, cyclic voltammetry, and photoluminescence in the presence of applied current.

Recipes for enhanced molecular cooling.

Abstract: Molecular nanomagnets are considered valid candidates for magnetic refrigeration at low temperatures. Designing these materials for enhanced cooling requires the control and optimization of the quantum properties at the molecular level, in particular: spin ground state, magnetic anisotropy, and presence of low-lying excited spin states. Herein, we present the theoretical framework together with a critical review of recent results, and perspectives for future developments.

Three-dimensional roton excitations and supersolid formation in Rydberg-excited Bose-Einstein condensates.

Abstract: We study the behavior of a Bose-Einstein condensate in which atoms are weakly coupled to a highly excited Rydberg state. Since the latter have very strong van der Waals interactions, this coupling induces effective, nonlocal interactions between the dressed ground state atoms, which, opposed to dipolar interactions, are isotropically repulsive. Yet, one finds partial attraction in momentum space, giving rise to a roton-maxon excitation spectrum and a transition to a supersolid state in three-dimensional condensates. A detailed analysis of decoherence and loss mechanisms suggests that these phenomena are observable with current experimental capabilities.

New parametrization for the nuclear covariant energy density functional with a point-coupling interaction

Abstract: A new parametrization PC-PK1 for the nuclear covariant energy density functional with nonlinear point-coupling interaction is proposed by fitting to observables of 60 selected spherical nuclei, including the binding energies, charge radii, and empirical pairing gaps. The success of PC-PK1 is illustrated in the description of infinite nuclear matter and finite nuclei including the ground-state and low-lying excited states. In particular, PC-PK1 provides a good description for the isospin dependence of binding energy along either the isotopic or the isotonic chain, which makes it reliable for application in exotic nuclei. The predictive power of PC-PK1 is also illustrated for the nuclear low-lying excitation states in a five-dimensional collective Hamiltonian in which the parameters are determined by constrained calculations for triaxial shapes.

On the Performances of the M06 Family of Density Functionals for Electronic Excitation Energies

Abstract: We assessed the accuracy of the four members of the M06 family of functionals (M06-L, M06, M06-2X, and M06-HF) for the prediction of electronic excitation energies of main-group compounds by time-dependent density functional theory. This is accomplished by comparing the predictions both to high-level theoretical benchmark calculations and some experimental data for gas-phase excitation energies of small molecules and to experimental data for midsize and large chromogens in liquid-phase solutions. The latter comparisons are carried out using implicit solvation models to include the electrostatic effects of solvation. We find that M06-L is one of the most accurate local functionals for evaluating electronic excitation energies, that M06-2X outperforms BHHLYP, and that M06-HF outperforms HF, although in each case, the compared functionals have the same or a similar amount of Hartree-Fock exchange. For the majority of investigated excited states, M06 emerges as the most accurate functional among the four tested, and it provides an accuracy similar to the best of the other global hybrids such as B3LYP, B98, and PBE0. For 190 valence excited states, 20 Rydberg states, and 16 charge transfer states, we try to provide an overall assessment by comparing the quality of the predictions to those of time-dependent Hartree-Fock theory and nine other density functionals. For the valence excited states, M06 yields a mean absolute deviation (MAD) of 0.23 eV, whereas B3LYP, B98, and PBE0 have MADs in the range 0.19-0.22 eV. Of the functionals tested, M05-2X, M06-2X, and BMK are found to perform best for Rydberg states, and M06-HF performs best for charge transfer states, but no single functional performs satisfactorily for all three kinds of excitation. The performance of functionals with no Hartree-Fock exchange is of great practical interest because of their high computational efficiency, and we find that M06-L predicts more accurate excitation energies than other such functionals.

Non-Abelian quantum order in spin-orbit-coupled semiconductors: Search for topological Majorana particles in solid-state systems

Abstract: We show that an ordinary semiconducting thin film with spin-orbit coupling can, under appropriate circumstances, be in a quantum topologically ordered state supporting exotic Majorana excitations which follow non-Abelian statistics. The key to the quantum topological order is the coexistence of spin-orbit coupling with proximity-induced $s$-wave superconductivity and an externally induced Zeeman coupling of the spins. For the Zeeman coupling below a critical value, the system is a nontopological (proximity-induced) $s$-wave superconductor. However, for a range of Zeeman coupling above the critical value, the lowest energy excited state inside a vortex is a zero-energy Majorana fermion state. The system, thus, has entered into a non-Abelian $s$-wave superconducting state via a topological quantum phase transition (TQPT) tuned by the Zeeman coupling. In the topological phase, since the time-reversal symmetry is explicitly broken by the Zeeman term in the Hamiltonian, the edge of the film constitutes a chiral Majorana wire. Just like the $s$-wave superconductivity, the Zeeman coupling can also be proximity induced in the film by an adjacent magnetic insulator. We show this by an explicit model tight-binding calculation for both types of proximity effects in the heterostructure geometry. Here we show that the same TQPT can be accessed by varying the interface transparency between the film and the superconductor. For the transparency below (above) a critical value, the system is a topological (regular) $s$-wave superconductor. In the one-dimensional version of the same structure and for the Zeeman coupling above the critical value, there are localized Majorana zero-energy modes at the two ends of a semiconducting quantum nanowire. In this case, the Zeeman coupling can be induced more easily by an external magnetic field parallel to the wire, obviating the need for a magnetic insulator. We show that, despite the fact that the superconducting pair potential in the nanowire is explicitly $s$ wave, tunneling of electrons to the ends of the wire reveals a pronounced zero-bias peak. Such a peak is absent when the Zeeman coupling is below its critical value, i.e., the nanowire is in the nontopological $s$-wave superconducting state. We argue that the observation of this zero-bias tunneling peak in the semiconductor nanowire is possibly the simplest and clearest experiment proposed so far to unambiguously detect a Majorana fermion mode in a condensed-matter system.

Singlet fission in pentacene through multi-exciton quantum states

Abstract: Multi-exciton generation-the creation of multiple charge carrier pairs from a single photon-has been reported for several materials and may dramatically increase solar cell efficiency. Singlet fission, its molecular analogue, may govern multi-exciton generation in a variety of materials, but a fundamental mechanism for singlet fission has yet to be described. Here, we use sophisticated ab initio calculations to show that singlet fission in pentacene proceeds through rapid internal conversion of the photoexcited state into a dark state of multi-exciton character that efficiently splits into two triplets. We show that singlet fission to produce a pair of triplet excitons must involve an intermediate state that (i) has a multi-exciton character, (ii) is energetically accessible from the optically allowed excited state, and (iii) efficiently dissociates into multiple electron-hole pairs. The rational design of photovoltaic materials that make use of singlet fission will require similar ab initio analysis of multi-exciton states such as the dark state studied here.

On the efficiency limit of triplet–triplet annihilation for photochemical upconversion

Abstract: Photochemical upconversion is performed, whereby emitter triplet states are produced through triplet energy transfer from sensitizer molecules excited with low energy photons. The triplet emitter molecules undergo triplet-triplet annihilation to yield excited singlet states which emit upconverted fluorescence. Experiments comparing the 560 nm prompt fluorescence when rubrene emitter molecules are excited directly, using 525 nm laser pulses, to the delayed, upconverted fluorescence when the porphyrin sensitizer molecules are excited with 670 nm laser pulses reveal annihilation efficiencies to produce excited singlet emitters in excess of 20%. Conservative measurements reveal a 25% annihilation efficiency, while a direct comparison between the prompt and delayed fluorescence yield suggests a value as high as 33%. Due to fluorescence quenching, the photon upconversion efficiencies are lower, at 16%.

Structure of even-even nuclei using a mapped collective Hamiltonian and the D1S Gogny interaction

Abstract: A systematic study of low energy nuclear structure at normal deformation is carried out using the Hartree-Fock-Bogoliubov theory extended by the generator coordinate method and mapped onto a five-dimensional collective quadrupole Hamiltonian. Results obtained with the Gogny D1S interaction are presented from drip line to drip line for even-even nuclei with proton numbers $Z=10$ to $Z=110$ and neutron numbers $N\ensuremath{\leqslant}200$. The properties calculated for the ground states are their charge radii, two-particle separation energies, correlation energies, and the intrinsic quadrupole shape parameters. For the excited spectroscopy, the observables calculated are the excitation energies and quadrupole as well as monopole transition matrix elements. We examine in this work the yrast levels up to $J=6$, the lowest excited ${0}^{+}$ states, and the two next yrare ${2}^{+}$ states. The theory is applicable to more than $90%$ of the nuclei that have tabulated measurements. We assess its accuracy by comparison with experiments on all applicable nuclei where the systematic tabulations of the data are available. We find that the predicted radii have an accuracy of $0.6%$, much better than can be achieved with a smooth phenomenological description. The correlation energy obtained from the collective Hamiltonian gives a significant improvement to the accuracy of the two-particle separation energies and to their differences, the two-particle gaps. Many of the properties depend strongly on the intrinsic deformation and we find that the theory is especially reliable for strongly deformed nuclei. The distribution of values of the collective structure indicator ${R}_{42}=E({4}_{1}^{+})/E({2}_{1}^{+})$ has a very sharp peak at the value 10/3, in agreement with the existing data. On average, the predicted excitation energy and transition strength of the first ${2}^{+}$ excitation are $12%$ and $22%$ higher than experiment, respectively, with variances of the order of $40--50%$. The theory gives a good qualitative account of the range of variation of the excitation energy of the first excited ${0}^{+}$ state, but the predicted energies are systematically $50%$ high. The calculated yrare ${2}^{+}$ states show a clear separation between $\ensuremath{\gamma}$ and $\ensuremath{\beta}$ excitations, and the energies of the ${2}^{+}$ $\ensuremath{\gamma}$ vibrations accord well with experiment. The character of the ${0}_{2}^{+}$ state is interpreted as shape coexistence or $\ensuremath{\beta}$-vibrational excitations on the basis of relative quadrupole transition strengths. Bands are predicted with the properties of $\ensuremath{\beta}$ vibrations for many nuclei having ${R}_{42}$ values corresponding to axial rotors, but the shape coexistence phenomenon is more prevalent. The data set of the calculated properties of 1712 even-even nuclei, including spectroscopic properties for 1693 of them, are provided in CEA Web site and EPAPS repository with this article [1].

Collective Lamb shift in single-photon superradiance.

Abstract: An atom, when excited, will typically decay with a characteristic decay time. An ensemble of atoms, collectively coupled together with just one of the atoms excited will conspire to decay much faster than the single atom case. This enhancement of light-matter interaction is known as superradiance. Rohlsberger et al. (p. [1248][1], published online 13 May; see the cover; see the Perspective by [Scully and Svidzinsky][2] ) present the realization of an artificial superradiant system comprising resonant iron atoms embedded in a semiconductor cavity and excited by synchrotron radiation and report the signature collective Lamb shift expected from the cooperative interaction and enhanced decay rate. The availability of such a controlled system to look closer at this effect should shed light on its role in natural and complex light-harvesting systems, and possibly allow the production of more efficient solar cells. [1]: /lookup/doi/10.1126/science.1187770 [2]: /lookup/doi/10.1126/science.1190737

Luminescent Ag7 and Ag8 Clusters by Interfacial Synthesis

Abstract: Molecular quantum clusters of noble metals are a fascinating area of contemporary interest in nanomaterials. While Au11, [1] Au13, [2] and Au55 [3] have been known for a few decades, several new clusters were discovered recently. These include Au8, [4] Au18, [5] Au25, [6] Au38, [7] and so on. Au11 has also been the subject of recent research. In view of their luminescence, several of these clusters are expected to be important in biolabeling and fluorescence resonance energy transfer as well as for creating luminescent patterns. There are many examples of template-assisted synthesis of water-soluble luminescent silver clusters with cores ranging from Ag2 to Ag8, having characteristic electronic transitions between 400–600 nm. However, unlike the case of gold, there are only limited examples of monolayer-protected silver analogues. Silver clusters protected with aryl, aliphatic, and chiral thiols have been reported, some of which have characteristic optical and mass spectrometric signatures. There is also a family of well-characterized metal-rich silver chalcogenide clusters. Besides single-crystal diffraction, mass spectrometry has also been used for detailed understanding of these clusters. Ag clusters with and without luminescence have also been reported. Herein we present gram-scale syntheses of two luminescent silver clusters, protected by small molecules containing thiol groups, with well-defined molecular formulas, by interfacial synthesis. This new synthetic approach has become promising in several other areas including semiconductor nanoparticles, two-dimensional superlattices, and 3D structures. A crude mixture of redand blue-green-emitting clusters Ag8(H2MSA)8 and Ag7(H2MSA)7 (H2MSA: mercaptosuccinic acid), respectively, was synthesized in gram quantities by an interfacial etching reaction conducted at an aqueous/ organic interface starting from H2MSA-protected silver nanoparticles (Ag@H2MSA) [19] as precursor (for details see the Experimental Section and Figure S1 in the Supporting Information). During the reaction, the optical absorption spectrum of the aqueous phase showed gradual disappearance of the surface plasmon resonance at 400 nm (Figure 1A) of metallic silver nanoparticles. The color of the aqueous phase gradually changed from brown to yellow and finally to orange. The particles of Ag@MSA are polydisperse (Figure 1Ca) and form smaller clusters in the aqueous phase upon etching (Figure 1Cb) with complete disappearance of the nanoparticles. The unetched particles move to the junction of the two phases and form a self-assembled film of monodisperse nanoparticles, resembling two-dimensional superlattices (Figure 1Cc), which appears blue in color. The smaller clusters formed in the reaction upon longer electron-beam irradiation coalesce to form nanoparticles (Figure S2). It is known that such clusters are unstable to high-energy electrons. The peak at 600 nm, which appears at shorter reaction time (60 min) and may be due to interplasmon coupling, disappears slowly, and a new feature is seen at 550 nm after 48 h of reaction (Figure 1A). In accordance with previous studies on silver clusters, we assign this peak to interband Figure 1. A) Time-dependent UV/Vis spectra of the clusters synthesized during interfacial etching at room temperature. B) UV/Vis absorption spectra of the clusters obtained from the two bands in PAGE. The inset shows a photograph of the wet gel after electrophoresis in UV light at room temperature, and the inset to the inset an image of the first band at 273 K. C) HRTEM images of a) assynthesized Ag@(H2MSA), b) the product obtained after interfacial etching, and c) particles in the blue layer at the interface. Individual clusters are not observable by TEM, but aggregates are seen faintly (b, shown in circles). Insets of (a) and (b) are photographs of Ag@MSA and crude cluster samples. d) Photographs of aqueous of cluster solutions of first (cluster 1) and second (cluster 2) PAGE bands at 273 K and room temperature, respectively. D) Luminescence emission of cluster 1 and cluster 2 in water, excited at 550 and 350 nm, respectively.

Theory of excited state decays and optical spectra: application to polyatomic molecules.

Abstract: General formalism of absorption and emission spectra, and of radiative and nonradiative decay rates are derived using a thermal vibration correlation function formalism for the transition between two adiabatic electronic states in polyatomic molecules. Displacements, distortions, and Duschinsky rotation of potential energy surfaces are included within the framework of a multidimensional harmonic oscillator model. The Herzberg−Teller (HT) effect is also taken into account. This formalism gives a reliable description of the Qx spectral band of free-base porphyrin with weakly electric dipole-allowed transitions. For the strongly dipole-allowed transitions, e.g., S1 → S0 and S0 → S1 of linear polyacenes, anthracene, tetracene, and pentacene, the HT effect is found to enhance the radiative decay rates by ∼10% compared to those without the HT effect. For nonradiative transition processes, a general formalism is presented to extend the application scope of the internal conversion theory by going beyond the promo...

Energy transfer dynamics in metal-organic frameworks

Abstract: Isomorphous metal−organic frameworks (MOFs) based on {M[4,4′-(HO2C)2-bpy]2bpy}2+ building blocks (where M = Ru or Os) were designed and synthesized to study the classic Ru to Os energy transfer process that has potential applications in light-harvesting with supramolecular assemblies. The crystalline nature of the MOFs allows precise determination of the distances between metal centers by X-ray diffraction, thereby facilitating the study of the Ru→Os energy transfer process. The mixed-metal MOFs with 0.3, 0.6, 1.4, and 2.6 mol % Os doping were also synthesized in order to study the energy transfer dynamics with a two-photon excitation at 850 nm. The Ru lifetime at 620 nm decreases from 171 ns in the pure Ru MOF to 29 ns in the sample with 2.6 mol % Os doping. In the mixed-metal samples, energy transfer was observed with an initial growth in Os emission corresponding with the rate of decay of the Ru excited state. These results demonstrate rapid, efficient energy migration and long distance transfer in iso...

πσ* excited states in molecular photochemistry

Abstract: The last few years have seen a surge in interest (both theoretical and experimental) in the photochemistry of heteroaromatic molecules (e.g. azoles, phenols), which has served to highlight the importance of dissociative excited states formed by electron promotion to σ* molecular orbitals. Such excited states—which, for brevity, are termed πσ* states in this Perspective article—may be populated by direct photo-excitation (though the transition cross-sections are intrinsically small), or indirectly, by non-adiabatic coupling from an optically ‘bright’ excited state (e.g. an excited state resulting from π* ← π excitation). The analogous πσ* excited states in prototypical hydride molecules like H2O and NH3 have long been recognised. They have served as test-beds for developing concepts like Rydbergisation, conical intersections (CIs) between potential energy surfaces, and for investigating the ways in which non-adiabatic couplings at such CIs influence the eventual photofragmentation dynamics. This Perspective article seeks to highlight the continuity of behaviour revealed by the earlier small molecule studies and by the more recent studies of heteroaromatic systems, and to illustrate the photochemical importance of πσ* excited states in many broad families of molecules. Furthermore, the dynamical influence of such excited states is not restricted to closed shell species; the Article concludes with a brief consideration of the consequences of populating σ* orbitals in free radical species, in molecular cations, and in dissociative electron attachment processes.

Reductive side of water splitting in artificial photosynthesis: new homogeneous photosystems of great activity and mechanistic insight.

Abstract: Rhodamine photosensitizers (PSs) substituting S or Se for O in the xanthene ring give turnover numbers (TONs) as high as 9000 for the generation of hydrogen via the reduction of water using [CoIII(dmgH)2(py)Cl] (where dmgH = dimethylglyoximate and py = pyridine) as the catalyst and triethanolamine as the sacrificial electron donor. The turnover frequencies were 0, 1700, and 5500 mol H2/mol PS/h for O, S, and Se derivatives, respectively (ΦH2 = 0%, 12.2%, and 32.8%, respectively), which correlates well with relative triplet yields estimated from quantum yields for singlet oxygen generation. Phosphorescence from the excited PS was quenched by the sacrificial electron donor. Fluorescence lifetimes were similar for the O- and S-containing rhodamines (∼2.6 ns) and shorter for the Se analog (∼0.1 ns). These data suggest a reaction pathway involving reductive quenching of the triplet excited state of the PS giving the reduced PS− that then transfers an electron to the Co catalyst. The longer-lived triplet state ...

Quantum size effects in ambient CO oxidation catalysed by ligand-protected gold clusters

Abstract: Gold nanoparticles can catalyse oxidation reactions in remarkably mild conditions and have excited much interest in recent years. With experimental studies disagreeing over the size of the most active nanoparticles, density functional calculations have now shown that limiting the particle size to below two nanometres is crucial.

Strongly correlated gases of Rydberg-dressed atoms: quantum and classical dynamics.

Abstract: We discuss techniques to generate long-range interactions in a gas of ground state alkali atoms, by weakly admixing excited Rydberg states with laser light. This provides a tool to engineer strongly correlated phases with reduced decoherence from inelastic collisions and spontaneous emission. As an illustration, we discuss the quantum phases of dressed atoms with dipole-dipole interactions confined in a harmonic potential, as relevant to experiments. We show that residual spontaneous emission from the Rydberg state acts as a heating mechanism, leading to a quantum-classical crossover.

Excited state dynamics in solid and monomeric tetracene: The roles of superradiance and exciton fission

Abstract: The excited state dynamics in polycrystalline thin films of tetracene are studied using both picosecond fluorescence and femtosecond transient absorption. The solid-state results are compared with those obtained for monomeric tetracene in dilute solution. The room temperature solid-state fluorescence decays are consistent with earlier models that take into account exciton-exciton annihilation and exciton fission but with a reduced delayed fluorescence lifetime, ranging from 20–100 ns as opposed to 2 μs or longer in single crystals. Femtosecond transient absorption measurements on the monomer in solution reveal several excited state absorption features that overlap the ground state bleach and stimulated emission signals. On longer timescales, the initially excited singlet state completely decays due to intersystem crossing, and the triplet state absorption superimposed on the bleach is observed, consistent with earlier flash photolysis experiments. In the solid-state, the transient absorption dynamics are ...

Measurement of Fast Electron Spin Relaxation Times with Atomic Resolution

Abstract: Single spins in solid-state systems are often considered prime candidates for the storage of quantum information, and their interaction with the environment the main limiting factor for the realization of such schemes. The lifetime of an excited spin state is a sensitive measure of this interaction, but extending the spatial resolution of spin relaxation measurements to the atomic scale has been a challenge. We show how a scanning tunneling microscope can measure electron spin relaxation times of individual atoms adsorbed on a surface using an all-electronic pump-probe measurement scheme. The spin relaxation times of individual Fe-Cu dimers were found to vary between 50 and 250 nanoseconds. Our method can in principle be generalized to monitor the temporal evolution of other dynamical systems.

Surface Plasmon Mediated Strong Exciton−Photon Coupling in Semiconductor Nanocrystals

Abstract: We present an experimental demonstration of strong coupling between a surface plasmon propagating on a planar silver thin film and the lowest excited state of CdSe nanocrystals Attenuated total reflection measurements demonstrate the formation of plasmon−exciton mixed states, characterized by a Rabi splitting of ∼112 meV at room temperature Such a coherent interaction has the potential for the development of nonlinear plasmonic devices, and furthermore, this system is akin to those studied in cavity quantum electrodynamics, thus offering the possibility to study the regime of strong light−matter coupling in semiconductor nanocrystals under easily accessible experimental conditions

CdSe Quantum Dots for Two-Photon Fluorescence Thermal Imaging

Abstract: The technological development of quantum dots has ushered in a new era in fluorescence bioimaging, which was propelled with the advent of novel multiphoton fluorescence microscopes. Here, the potential use of CdSe quantum dots has been evaluated as fluorescent nanothermometers for two-photon fluorescence microscopy. In addition to the enhancement in spatial resolution inherent to any multiphoton excitation processes, two-photon (near-infrared) excitation leads to a temperature sensitivity of the emission intensity much higher than that achieved under one-photon (visible) excitation. The peak emission wavelength is also temperature sensitive, providing an additional approach for thermal imaging, which is particularly interesting for systems where nanoparticles are not homogeneously dispersed. On the basis of these superior thermal sensitivity properties of the two-photon excited fluorescence, we have demonstrated the ability of CdSe quantum dots to image a temperature gradient artificially created in a biocompatible fluid (phosphate-buffered saline) and also their ability to measure an intracellular temperature increase externally induced in a single living cell.

Triplet States and Electronic Relaxation in Photoexcited Graphene Quantum Dots

Abstract: Electronic relaxation in photoexcited graphenes is central to their photoreactivity and their optoelectrical applications such as photodetectors and solar cells. Herein we report on the first ensemble studies of electronic energy relaxation pathways in colloidal graphene quantum dots with uniform size. We show that the photoexcited graphene quantum dots have a significant probability of relaxing into triplet states and emit both phosphorescence and fluorescence at room temperature, with relative intensities depending on the excitation energy. Because of the long lifetime and reactivity of triplet electronic states, our results could have significant implications for applications of graphenes.

Generation of molecular hot electroluminescence by resonant nanocavity plasmons

Abstract: Control of the radiative properties of functional molecules near metals is a key issue in nano-optics, and is particularly important in the fields of energy transfer and light manipulation at the nanoscale1,2 and the development of plasmonic devices3,4,5. Despite the various vibronic transitions (S1(v′) → S0(v)) available for frequency tuning of fluorescence, the molecular emissions near metals reported to date have been subject to Kasha's rule, with radiative decay from the lowest excited state (S1(0)) (refs 6–10). Here, we show resonant hot electroluminescence arising directly from higher vibronic levels of the singlet excited state (S1(v′ > 0)) for porphyrin molecules confined inside a nanocavity in a scanning tunnelling microscope, by spectrally tuning the frequency of plasmons. We also demonstrate the generation of unexpected upconversion electroluminescence. These observations suggest that the local nanocavity plasmons behave like a strong coherent optical source with tunable energy, and can be used to actively control the radiative channels of molecular emitters by means of intense resonance enhancement of both excitation and emission. Nanocavity plasmons are exploited as a coherent optical source with tunable energy and to actively control the radiative channels of molecules. Intense resonance enhancement of both excitation and emission, in an effect called resonant hot-electroluminescence, is demonstrated for porphyrin molecules confined inside a nanocavity.